Human chorionic gonadotropin (hereinafter abbreviated as hCG) is a kind of protein hormone which is produced from trophoblastic cells formed upon pregnancy, and it promotes secretion of progesterone. The detection of hCG is generally utilized for a primary diagnosis of pregnancy. Further, in chorionic diseases such as hydatid mole, destructive mole, choriocarcinoma and the like, it has been found that the determination of hCG in body fluids such as urine, blood and the like is of extreme importance from the viewpoints of early discovery, judgment of therapeutic effects and prognostic control of these diseases. However, in order to diagnose these diseases, it is necessary to determine a very small amount of hCG such as about 100 IU/liter or less. In this respect, a problem is immunological cross reactions with protein hormones having structures similar to that of hCG, namely, luteinizing hormone (hereinafter sometimes abbreviated as hLH), follicle-stimulating hormone (hereinafter sometimes abbreviated as hFSH) and thyroid-stimulating hormone (hereinafter sometimes abbreviated as hTSH). Particularly, hLH has very high similarity to hCG and a hLH concentration in urine sometimes reaches 150 IU/liter depending upon physiological conditions. Therefore, in order to determine hCG in a small amount of a body fluid, it is necessary to immunologically distinguish hCG from hLH.
On the other hand, according to the chemical analysis of these protein hormones, it has been found that the cross reactivity is due to their respective .alpha.-subunit parts which have many common structures. Thus, attempts have been made to specifically detect hCG by isolating and purifying .beta.-subunit of hCG (hereinafter sometimes abbreviated as hCG-.beta.), which has relatively less similarity, and producing an anti-hCG-.beta. antibody.
However, the cross reactivity to hLH can not be completely eliminated by using the anti-hCG-.beta. antibody because there still exists a common amino acid sequence in both .beta.-subunits of hCG and hLH. On the other hand, a peptide composed of about 30 amino acids located at the C-terminal of hCG-.beta. has an amino acid sequence which does not exist in hLH and it has become possible to completely distinguish hCG from hLH by utilizing this part. An antibody to the C-terminal region of hCG-.beta. reacts with only hCG-.beta., and it does not undergo cross reaction with hLH. Further, as an advance in studies on hCG-.beta., it has been presumed that a hCG-.beta. core region (hereinafter sometimes abbreviated as .beta. core) must be present. Namely, it has been shown that .beta. core is a glycoprotein composed of two peptides bound to each other through disulfide bond(s), one peptide of which is composed of the 6th to 40th amino acid residues of human chorionic gonadotropin .beta. subunit (hCG-.beta.) and the other is composed of the 55th to 92nd amino acid residues of hCG-.beta., and having a molecular weight of 12 to 17 kilodaltons (Endocrinology, 123, 572, 1988), and is present in urine of patients with not only obstetrical and gynecological malignant tumors such as ovarian cancer, cervical cancer or corpus utericancer but also various malignant tumors such as stomach cancer, kidney cancer and the like J. Clin. Endocrinol. Metab., 53, 1014, 1981; Cancer, 45, 2583, 1980; Acta Endocrinol. (Copenh), 112, 415, 1986!. However, there is no established theory of its production, secretion and physiological significance, and there is no biological determination method of .beta. core. Although a radioimmunoassay (RIA) using a polyclonal anti-hCG-.beta. antibody referred to as SB6 has been established (Am. J. Obstet. Gynecol., 113,751, 1972) as an immunological determination method, .beta. core can not be distinguished from human chorionic gonadotropin and hCG-.beta. because there is no difference between the affinity of the antibody to .beta. core and that to human chorionic gonadotropin and hCG-.beta..
Recently, monoclonal antibodies B202 and B204 for recognizing .beta. core have been prepared by using .beta. core as an immunogen (Endocrinology, 123, 584, 1988) and radioimmunoassay of .beta. core has been reported (Cancer Res., 48, 1361, 1988; Cancer Res., 48, 1356, 1988).
However, SB6 is a polyclonal antibody and, when it is used in a radioimmunoassay, the antibody has similar affinity for .beta. core, hCG and hCG-.beta., and has about 10% of cross reactivity to hLH.
B202, B204 and the like are monoclonal antibodies obtained by immunizing with .beta. core. A. Krichevsky et al. have prepared several monoclonal antibodies to .beta. core and have reported their properties in Endocrinology, 123, 584-593, 1988. However, in this paper, they have clearly described that no antibody to .beta. core has been obtained in the case of using hCG-.beta. as an immunogen.
On the other hand, in the field of clinical test, a novel monoclonal antibody to .beta. core having a high affinity, which enables a high sensitivity assay, has been requested.
In addition, in the conventional RIA of .beta. core, the assay system is assembled to determine only a urine specimen, and it is insufficient to determine a humoral tissue fluid, a tissue extract fluid and a tissue culture supernatant which are essential to study biosynthesis, secretion, distribution, metabolism and physiobiological effects of .beta. core as well as relation to diseases and the like.